Compressed air-driven general purpose reciprocating piston material pumps may be damaged as a result of an empty supply container, interrupted material supply to the pump, ruptured hoses and excessive cavitation. There have in the past been provided runaway valves that automatically shut off the supply of compressed air to the pump in response to an air flow rate in excess of a predetermined flow rate in an effort to protect the pump from damage as a result of these causes.
Such runaway valves are usually positioned in the supply air line between a pressure regulator and a piston reversing valve in the pump. All of the undesirable conditions noted above, i.e. empty supply container, interrupted material supply to the pump, ruptured hoses or excessive cavitation all result in a drop in pressure downstream from the runaway valve. Hence these runaway valves are designed to close when the pressure drop across the valve exceeds a predetermined maximum.
However, since it is desirable to change the flow rate through the valve to control pump speed, this complicates the runaway valve construction since flow rate is proportional to pressure drop and hence flow rate variation undesirably changes the responsiveness of the runaway valve.
In these prior runaway valves, after the valve closes in response to a predetermined downstream pressure drop, the valve remains in its closed position even though the condition causing the pressure drop has been corrected. Hence, a reset mechanism of some type is provided in the runaway valve to open the valve member after the downstream condition has been corrected. Usually this reset mechanism connects the inlet side of the valve to the outlet side across the poppet valve member and when downstream pressure approaches upstream pressure, a coil spring biasing the valve member moves it to its open position. This reset mechanism is slow-acting because the poppet valve member will not open until downstream pressure equals supply pressure, or approximately so, and hence the operator sometimes has to hold the reset mechanism for many seconds to be certain the valve member has been opened (remembering that he cannot visually see poppet valve member opening).
Still another disadvantage in prior runaway valve mechanisms is that they are subject to transient upstream pressure surges and temporary downstream pressure losses. If the runaway valve experiences a transient upstream pressure increase or a temporary downstream pressure loss, and produces a pressure drop across the valve member above the predetermined value, the poppet valve member will move to a closed position and shut the pump down unnecessarily, and of course the operator then is required to manually reset the runaway valve to again connect supply air to the material pump and reinstate its operation.
It is the primary object of the present invention to ameliorate the problems noted above in runaway valves for material pumps.